Plastic-clad optical fibers comprising quartz, silica, glass, etc. as a core and plastics as a cladding (hereinafter abbreviated as PCF) are relatively cheap, excellent in light transmission, and easy to have an increased numerical aperture and are therefore used as optical fibers for short-to-medium distance communication or light guides.
While silicone resins have conventionally been used as cladding materials, fluorine-containing resins having high hardness have recently been proposed and practically used as cladding materials from the standpoint of easy handling and environmental resistance as disclosed in U.S. Pat. Nos. 4,511,209 and 4,707,076, JP-A-63-40104, JP-A-63-43104, JP-A-63-208805, JP-A-63-208806, JP-A-63-208807,JP-A-63-249112, EP 257863, and EP 333464 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") In particular, EP 257863 discloses an active energy ray-curing cladding material mainly comprising fluorinated acrylates and an optical fiber using the composition as a cladding material, in which the curable cladding material comprises a fluorine-containing curing monomer containing a fluorinated alkyl group having not less than 6 carbon atoms, a fluorine-containing curing monomer containing a fluorinated alkyl group having not more than 5 carbon atoms, and a polyfunctional curing monomer with a weight ratio of the former fluorine-containing curing monomer to the latter fluorine-containing curing monomer being 68/32.
However, having poor compatibility or homogeneity at room temperature, the above-described curable cladding material, when coated as such at room temperature, provides optical fibers having seriously deteriorated optical characteristics such as light transmission properties. Besides, the formed cladding has poor adhesion to the core and easily peels off, causing reduction of environmental resistance or tensile strength of the optical fiber, eventually making the optical fiber useless. Where the cladding material is heated for coating so as to have improved compatibility or homogeneity, the heating temperature must be strictly controlled to prevent eccentricity, etc., which accordingly requires a complicated drawing apparatus and deteriorates workability. Further, where the cladding resin is rendered transparent at room temperature, the resulting cladding layer would have reduced mechanical strength and an increased refractive index and fail to maintain a desired numerical aperture.
Therefore, under the present situation, there is no cladding material which exhibits satisfactory transparency at room temperature, has a low refractive index and excellent workability, and exhibits excellent transparency and dynamic strength after curing to thereby provide an optical fiber having sufficient dynamic strength, optical characteristics, and environmental resistance, e.g., heat- and humidity-resistance.